The objective of this study was to determine the frequency and clinical characteristics of selective IgA deficiency (SIgAD) in children and adults with systemic lupus erythematosus (SLE), and evaluate potential differences in presentation and course of the SLE. IgA deficiency was defined as a serum IgA concentration < or =0.01 mg/mL determined on two sera by radial diffusion. SLE was classified by the 1982 criteria of the American College of Rheumatology. Seventy-seven children with SLE followed prospectively for > or =20 years and 152 adults surveyed during a one-year period were assayed for serum IgA levels. Disease characteristics were compared among the deficient patients and the IgA-normal patients. Twelve patients with SIgAD were identified: 1) Juvenile(J)-SLE: four children with juvenile onset (< or =18 years) and four others encountered as adults; and 2) Adult(A)-SLE: four patients with adult onset. No significant differences were found in clinical presentation or course except for a possible increase in recurrent infections and the observation that there were only two African-Americans. Five patients had received blood transfusions with no reactions; three of these patients had serum anti-IgA antibodies. One pediatric patient developed low levels of IgA (